Wednesday, May 12, 2021

Cicadas? - Not Here (eastern MA)

Shell left behind after the cicada emerges from
underground, splits the back and flies away.
For 17 years, cicadas of the infamous cicada swarms do very little, and none of it above ground. They suck sugar out of tree roots. Then, following this absurdly long subterranean life, they emerge, sprout wings, make a ton of noise, have sex, lay eggs and die.

“Brood X” is emerging from its 17-year underground sequester this year, but not here. Brood XIII will emerge in 2024, but not here. Brood XIV will emerge in 2025, but not here. According to a map created by the U.S. Department of Agriculture Forest Service, identifying 15 periodical cicada broods that affect (afflict?) the United States, none of them are present in the greater Boston area. However, if you intend to visit parts of New Jersey, Pennsylvania, Delaware, Maryland, Ohio, Indiana, Kentucky, North Carolina or Tennessee this May or June, you will be sharing the great outdoors with billions upon billions of their discarded nymph shells, the noisy cicadas, and then their carcasses once they all die. And by noisy, meaning four to six weeks of lawnmower-loud males, all making a high-pitched buzz in their attempts to attract females. Note how different this is from moths, wherein the females release pheromones into the air, and males detect those molecules with their antennae.    

Cicadas mating
World-wide, there are more than 3,000 species of cicadas. Locally, the annual cicada actually has a life cycle of 2-5 years underground as a nymph before emerging to become a dark-bodied, dark-eyed adult, during late July and August. Although these take several years to mature there is no synchronization, so numbers are similar every summer, and the term “annual cicadas” is commonly used. Only in the eastern United States are there also the “periodical cicadas” which synchronize to either every 17 or 13 years (duration different for different species within the genus Magicicada).  

The life cycle of periodic cicadas has two mysteries: Why? and How? Long life cycles may have developed as a response to predators, such as the cicada killer wasp or praying mantis. The thinking is that specialist predators with a shorter life cycle could not reliably prey upon the cicadas, hence their population crashes during the following years. Generalist predators, such as many types of birds, snakes, racoons and skunks would in theory be subject to saturation of predator capacity, meaning that the predators at normal population density would eat all they could, yet not hinder breeding success. Skipping a small number of years between broods would not improve survival, as long-lived predators can increase in population right after the boom year, with those litters present in the immediately following years.

“How” is a mystery. First year, nymphs are not far below the surface, but after a few years of growth they are feeding from roots more than two feet below the surface, meaning that they are not getting any change of daylight duration or change of temperature signals. One speculation is that seasonal changes in the composition of root fluids are detected, but that still does not answer how the insects count to 17 (or 13). Another possibility is that the nymphs have an intrinsic molecular timing mechanism, such as ever-shortening DNA telomere length. Helping to keep the broods synchronized is the fact that those that emerge a year too soon or a year too late – having lost the count – are subject to severe predation, thus prevented from creating a new periodic population. Synchronizing the actual emergence appears to be linked to soil temperature, as global warming has shifted the timing a week or so earlier.

U.S. Forest Service Brood map. Brood X in yellsw
One way to think about the brood is as an acute increase in a protein- and fat-rich food appearing in a short period of time. Captured alive or scavenged after dead, these cicadas are delivering roughly 300 pounds (dried weight) per acre. Because it’s more than one million bugs per acre! Research shows that many bird species have better chick survival in brood years, followed by a population crash after.

Researchers recently solved one puzzle, which is how the cicada nymphs survive on nutritionally poor tree root fluid. Turns out, like termites, cicadas have symbiotic bacteria (in some species, fungi) that reside in their intestines, synthesizing the amino acids and vitamins their hosts need to survive. When the female creates eggs, the species in her gut microbiome are incorporated into the eggs.    

A color map showing all of the broods is available from the U.S. Forest Service

Although there are dozens of cicada recipes, Mark says he is not going to start to eat bugs.

NOT IN THE NEWSPAPER COLUMN: There are actually three species of 17-year cicadas, with different sounds. Males cluster in a 'chorus of cicadas' so that collectively their sound is louder. Females fly toward the sound. There is some evidence, that females select larger males. Females mate once. Males can mate more than once. 

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